It was formerly thought that Mercury possessed a mass greatly in excess of that which its size would seem to imply, and some estimates, based upon the apparent effect of its attraction on comets, made it equal in mean density to lead, or even to the metal mercury.  This led to curious speculations concerning its probable metallic composition, and the possible existence of vast quantities of such heavy elements as gold in the frame of the planet.  But more recent, and probably more correct, computations place Mercury third in the order of density among the members of the solar system, the earth ranking as first and Venus as second.  Mercury’s density is now believed to be less than the earth’s in the ratio of 85 to 100.  Accepting this estimate, we find that the force of gravity upon the surface of Mercury is only one third as great as upon the surface of the earth—­i.e., a body weighing 300 pounds on the earth would weigh only 100 pounds on Mercury.

This is an important matter, because not only the weight of bodies, but the density of the atmosphere and even the nature of its gaseous constituents, are affected by the force of gravity, and if we could journey from world to world, in our bodily form, it would make a great difference to us to find gravity considerably greater or less upon other planets than it is upon our own.  This alone might suffice to render some of the planets impossible places of abode for us, unless a decided change were effected in our present physical organization.

One of the first questions that we should ask about a foreign world to which we proposed to pay a visit, would relate to its atmosphere.  We should wish to know in advance if it had air and water, and in what proportions and quantities.  However its own peculiar inhabitants might be supposed able to dispense with these things, to us their presence would be essential, and if we did not find them, even a planet that blazed with gold and diamonds only waiting to be seized would remain perfectly safe from our invasion.  Now, in the case of Mercury, some doubt on this point exists.

Messrs. Huggins, Vogel, and others have believed that they found spectroscopic proof of the existence of both air and the vapor of water on Mercury.  But the necessary observations are of a very delicate nature, and difficult to make, and some astronomers doubt whether we possess sufficient proof that Mercury has an atmosphere.  At any rate, its atmosphere is very rare as compared with the earth’s, but we need not, on that account, conclude that Mercury is lifeless.  Possibly, in view of certain other peculiarities soon to be explained, a rare atmosphere would be decidedly advantageous.